Field
The present disclosure relates generally to communication systems, and more particularly, to a method and apparatus for managing specific absorption rate (SAR) distribution to maintain radio connection while simultaneously providing bursts of high power.
Background
Wireless communication devices have become smaller and more powerful as well as more capable. Increasingly users rely on wireless communication devices for mobile phone use as well as email and Internet access. At the same time, devices have become smaller in size. Devices such as cellular telephones, personal digital assistants (PDAs), laptop computers, and other similar devices provide reliable service with expanded coverage areas. Such devices may be referred to as mobile stations, stations, access terminals, user terminals, subscriber units, user equipment, and similar terms.
A wireless communication system may support communication for multiple wireless communication devices at the same time. In use, a wireless communication device may communicate with one or more base stations by transmissions on the uplink and downlink. Base stations may be referred to as access points, Node Bs, or other similar terms. The uplink or reverse link refers to the communication link from the wireless communication device to the base station, while the downlink or forward link refers to the communication from the base station to the wireless communication devices.
Wireless communication systems may be multiple access systems capable of supporting communication with multiple users by sharing the available system resources, such as bandwidth and transmit power. Examples of such multiple access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, wideband code division multiple access (WCDMA) systems, global system for mobile (GSM) communication systems, enhanced data rates for GSM evolution (EDGE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Wireless devices, including mobile telephones, are required to undergo testing to determine the amount of RF energy a user may be exposed to when using the device. In the U.S., the Federal Communications Commission (FCC) certifies mobile devices to ensure compatibility with requirements and user safety. The maximum power that a mobile device may use when transmitting is affected by the fact that users position the device against their head and body. The close proximity or contact is behind the FCC requirements setting limits on the specific absorption rate (SAR). SAR is defined as the power absorbed per unit mass of tissue in mW/g by regulatory bodies, including the FCC. Current FCC testing requirements allow for a finite separation distance between the smartphone and the torso portion of a human phantom.
FCC certification of wireless devices requires SAR measurements be taken by attaching the wireless device to a phantom (representing the human body). This phantom is filled with a tissue simulating liquid. The required measurements are taken in five positions relative to the human body and produce five different SAR distributions. Additional SAR measurements are also required at multiple channels in a given frequency band for a particular antenna and transmitter combination. The FCC reviews the data from all positions and channels, resulting in the reporting of hundreds of measurements for all bands, transmitter, and antenna combinations.
The testing is used to determine maximum transmitter power for the band, transmitter, and antenna combination. The result may be a limitation on transmitter power. Current FCC testing allows for a finite separation distance between the mobile device and the flat body phantom. As the use of smartphones increases and the devices used in pockets and worn on the body, the FCC may require SAR testing with zero separation distance. This will drastically reduce the maximum allowable transmit power.
The testing procedures described above result in a SAR limit based on the maximum average power. However, the current compliance test procedure effectively results in enforcing an instantaneous power-based SAR, which results in an overly conservative power limit. This poses a potential roadblock for new technology development.
SAR is directly proportional to transmit power. Since the time-averaged transmit power should be less than the regulatory limit, a transmitter may only transmit at high power for short bursts of time and once the SAR limit is reached the call may be dropped. There is a need in the art for a method of managing SAR to allow a transmitter to reserve some power to maintain the radio connection while simultaneously providing bursts of high power while complying with regulatory SAR limits.